1. Field of the Invention
The present invention relates to an exhaust emission control device and method for an internal combustion engine, and an engine control unit, for purifying i.e. decreasing exhaust emissions by temporarily trapping NOx contained in exhaust gases discharged from the engine and performing reduction of the trapped NOx.
2. Description of the Related Art
Conventionally, there has been disclosed an exhaust emission control device for an internal combustion engine, e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2006-207487. This internal combustion engine is a diesel engine in which a three-way catalyst and a NOx catalyst are disposed in an exhaust pipe thereof from the upstream side in the mentioned order, for purifying HC and CO mainly during low temperature, and for eliminating harmful NOx ingredients, i.e. purifying NOx, respectively. In this exhaust emission control device, exhaust gases discharged from the engine during lean operation thereof pass through the three-way catalyst and then flow into the NOx catalyst, wherein NOx in the exhaust gases is trapped.
When the amount of trapped NOx reaches a predetermined value, rich spike is executed to cause the air-fuel ratio of a mixture supplied to the engine is controlled to a richer side than a stoichiometric air-fuel ratio. The rich spike causes unburned fuel components in the exhaust gases to be supplied to the NOx catalyst as reducing agent, whereby the NOx trapped in the NOx catalyst is reduced to be purified, i.e. changed into harmless ingredients. Further, the reducing agent supply amount indicative of the amount of reducing agent supplied to the NOx catalyst is calculated based on the detected air-fuel ratio detected by an air-fuel ratio sensor disposed at a location upstream of the three-way catalyst, the space velocity of exhaust gases, etc., and when the cumulative value of the reducing agent supply amount exceeds a threshold value set according to the trapped NOx amount, it is judged that the reduction of NOx is complete, so that the rich spike is terminated.
As described above, according to this exhaust emission control device, since the three-way catalyst is disposed upstream of the NOx catalyst, the reducing agent supplied by rich spike is consumed to some extent in the three-way catalyst, and is then supplied to the NOx catalyst. Specifically, in the three-way catalyst, reducing agent is oxidized and consumed by oxygen stored therein before the start of the rich spike, and hence the amount of reducing agent actually supplied to the NO catalyst becomes smaller accordingly. Further, the consumption amount of reducing agent varies with the degree of degradation of the three-way catalyst, and as the degree of degradation is higher, the oxygen storage capability and oxidation capability are lowered, and hence the consumption amount of reducing agent is reduced.
In the conventional exhaust emission control devices, however, the facts that reducing agent is consumed in the three-way catalyst, and that the reducing agent consumption amount varies with the degree of degradation of the three-way catalyst are not taken into consideration at all, but the amount of reducing agent supplied to the NOx catalyst is only calculated based on the detected air-fuel ratio of exhaust gases upstream of the three-way catalyst, etc. For this reason, when the amount of reducing agent consumed in the three-way catalyst is large, the amount of reducing agent supplied to the NOx catalyst is calculated to be larger than the actual amount, so that the cumulative value of the reducing agent supply amount exceeds the threshold value before the reduction of NOx is not actually complete to terminate the rich spike. This increases exhaust emissions due to insufficient reduction of NOx. Inversely, when the amount of reducing agent consumed in the three-way catalyst is small, the timing for termination of the rich spike becomes late than it should be, which causes an excessive supply of reducing agent. This causes excess amounts of CO and HC to be generated, resulting in increased exhaust emissions and degraded fuel economy.